1. Field of the Invention
The present invention relates to a method and a device for developing a latent image formed on an image carrier and an image forming apparatus using the same.
2. Description of the Background Art
An image forming apparatus of the type developing a latent image formed on an image carrier with a developer made up of toner grains and magnetic carrier grains is conventional and implemented as a copier, a printer or a facsimile apparatus by way of example, Generally, a developing device included in this type of image forming apparatus uses a rotatable sleeve, which accommodates stationary magnetic field forming means therein, as a developer carrier. The sleeve in rotation conveys the developer deposited thereon to a developing zone. In the developing zone, the developer forms a magnet brush around a position where the sleeve and image carrier are closest to each other, and contacts the image carrier. In the developing zone, the toner grains are caused to deposit on a latent image formed on the image carrier by an electric field, which is formed by the surface potential of the image carrier and a bias applied to the sleeve.
Various improvements relating to the developing device have heretofore been proposed to protect images from roughness or graininess. For example, the electric field between the image carrier and the sleeve may be implemented as an alternating electric field for effecting development while promoting the rearrangement of the toner grains. However, the maximum value of an alternating electric field is greater that the maximum value of a DC electric field, so that the carrier grains are apt to deposit on the image carrier. Also, an alternating electric field cannot be formed without resorting to an exclusive power supply, which increases cost. It is therefore desirable to obviate roughness while using a DC electric field.
Low density of the magnetic brush in the developing region is one of major causes of roughness and obstructs uniform development. In light of this, Japanese Patent Laid-Open Publication No. 8-146668, for example, proposes to determine the density of the magnet brush in the developing zone by using the volume ratio of carrier grains present in the developing zone.
We, however, experimentally found that graininess of an image was not constant for the same volume ratio of carrier grains. This means that even the volume ratio of carrier grains cannot account for the relation between the density of the magnet brush in the developing zone and graininess. In the case where a DC electric field is formed between the image carrier and the sleeve, toner grains fly from the tips of the magnet brush toward the image carrier, but fly from the roots of the magnet brush little. Stated another way, mainly the tips of the magnet brush contribute to development. It follows that consideration must be given to at least the arrangement and density of the magnet brush.